A two-component developer used in electrophotography is composed of a toner and a carrier. The carrier is a substance which is mixed and stirred with the toner in a developer box to give a desired charge to the toner and carries the charged toner onto an electrostatic latent image formed on a photoreceptor to form a toner image. The carrier is held by a magnet so as to remain on the developing roller after the toner image formation, and is made to return to the developer box where it is again mixed and stirred with fresh toner particles for repeated use over a certain period.
In contrast to the one-component developer, the two-component developer contains the carrier which has the functions such that the carrier stirs toner particles to give a desired charge to the toner particles and carries the toner. As a result, the two-component developer is excellent in controllability in designing developers, and hence is widely used in the fields of full color machines required to provide high image qualities and high-speed machines required to secure reliability and durability in image maintenance.
In such a two-component electrophotographic developer, for the purpose of obtaining high-quality images, particles of ferrites such as Cu—Zn ferrite and Ni—Zn ferrite have been used as carriers in place of oxide-coated iron powders and resin-coated iron powders. As compared to conventional iron powder carriers, ferrite carriers using such ferrite particles as described above are generally spherical and have many advantageous properties for obtaining high quality images such that the magnetic properties thereof are controllable. Further, resin-coated ferrite carriers prepared by coating such ferrite particles used as the carrier cores with various resins are improved in abrasion resistance, durability and the like, and make the volume resistivity controllable.
However, the conventional ferrite carriers containing heavy metals such as Cu, Zn and Ni currently tend to be avoided from the viewpoint of reduction of the load on the environment, regulations of waste, and others. Additionally, such heavy metal-containing ferrite carriers tend to have lower electrical resistance leading to a problem such that image defects are caused by leakage under high electric field. Further, there have been problems that such heavy metal-containing ferrite carriers are heavy in specific gravity and hardly capable of attaining high durability.
On the other hand, for the purpose of solving the above-described problems, some light metal-containing ferrite carriers have been disclosed. For example, Japanese Patent Laid-Open No. 2001-154416 describes an electrophotographic carrier using a magnesium-containing ferrite; additionally, Japanese Patent Laid-Open No. 7-225497 discloses a ferrite carrier for electrophotographic developer using a lithium-containing ferrite and a developer using the carrier; further, Japanese Patent Laid-Open No. 7-333910 discloses a ferrite carrier for electrophotographic developer using a lithium-containing ferrite which is partially substituted with an alkali earth metal oxide (MgO, CaO, SrO or BaO) and a developer using the carrier.
However, these light metal-containing ferrite carriers suffers from a problem such that the magnetization variation and electrical resistance variation among particles tend to be generated, and accordingly tend to cause image defects such as carrier adhesion when used in electrophotographic developers. Specifically, as disclosed in Japanese Patent Laid-Open No. 2001-154416, a ferrite carrier containing only magnesium as a main component degrades the saturation magnetization. On the other hand, when the saturation magnetization is enhanced by developing a magnetite phase through excessively mixing iron oxide and sintering in a reductive atmosphere, the electrical resistance is drastically decreased to induce carrier adhesion.
Additionally, as disclosed in Japanese Patent Laid-Open No. 7-225497, a ferrite carrier containing a large amount of lithium tends to give a large magnetization variation among particles.
Japanese Patent Laid-Open No. 7-333910 discloses a carrier for electrophotographic developer excellent in image quality and durability, benign to the environment, long in life and excellent in environmental stability, wherein the carrier is prepared by substituting part of Li2O and/or Fe2O3 in a lithium-containing ferrite with an alkali earth metal oxide such as MgO, CaO, SrO or BaO. However, such substitution creates a part to be a hard ferrite substituted with CaO, SrO or BaO, and consequently the remanent magnetization and the coercive force are increased to degrade the fluidity, and the surface property variation tends to be generated.
Additionally, in Example 2 and Example 3 in Japanese Patent Laid-Open No. 7-333910, examples of composite ferrites containing Li and Mg. As described below, lithium is a monovalent metal and Mg is a divalent metal, and hence the Li ferrite is present in a form of (Li2O) (Fe2O3)5 and the Mg ferrite is present in a form of (MgO) (Fe2O3); accordingly, in the ferrite of above Example 2, it is assessed that (Fe2O3) is present in excess by approximately 7 mol %.
Similarly, the composition of above Example 3 is assessed to contain (Li2O) in excess by approximately 1.9 mol %, or (Li2O) and (MgO) in excess by approximately 5.7 mol % in total.
Further, Japanese Patent Laid-Open No. 7-333910 discloses an example (Comparative Example 20) in which Mn is contained in a Li-containing ferrite; however, as shown in the evaluation results of Japanese Patent Laid-Open No. 7-333910, the scattered amount is abnormally large, so that such a ferrite is far from a practically usable level. However, in contrast to the present invention, Japanese Patent Laid-Open No. 7-333910 does not present any disclosure or suggestion with respect to the effect and the appropriate content range of Mn contained in a trace amount.
An attempt to solve the above-described problems has been made by containing a trace amount of a heavy metal element in these light metal-containing ferrite carriers. Japanese Patent Laid-Open No. 2006-154806 describes a carrier having a coating layer and a carrier core containing 10 to 40 mol % of a metal oxide containing at least one metal element selected from the group consisting of magnesium, lithium and calcium and containing 50 to 4000 ppm, in terms of the total sum, of a metal oxide containing at least one metal element selected from the group consisting of manganese, copper, chromium and zinc.
However, even by controlling the elements, described in Japanese Patent Laid-Open No. 2006-154806, used in wide ranges and the contents thereof, and the contents of trace components, the carriers thus obtained hardly fulfill the high-level requirements in the image formation using recent electrophotography. In Examples of Japanese Patent Laid-Open No. 2006-154806, the “carrier 1,” “carrier 13,” “carrier 14,” and “carrier 20” are disclosed as the carriers using the “carrier core 1.” As can be seen from the results obtained for Examples and Comparative Examples using these carriers, no sufficient properties can be obtained depending on the fine particles contained in the coating resin even by using the carrier core materials evaluated to have sufficiently satisfactory compositions in Japanese Patent Laid-Open No. 2006-154806.
For example, the “carrier core 1” in Japanese Patent Laid-Open No. 2006-154806 is a ferrite containing Li and Mg as main components and is described to be composed of LiO: 12.9 mol %, MgO: 6.5 mol % and Fe2O3: 80.6 mol %. As described above, lithium is a monovalent metal and Mg is a divalent metal, and hence the Li ferrite is present in a form of (Li2O) (Fe2O3)5 and the Mg ferrite is present in a form of (MgO) (Fe2O3); accordingly, even if LiO (divalent Li) is used to give the above described composition, it is assessed that (Fe2O3) is present in excess by approximately 42 mol %.
Similarly, the “carrier core 3” in Japanese Patent Laid-Open No. 2006-154806 leads to an assessment that (Li2O) is contained in excess by 6 mol %, or (Li2O) and (MgO) are contained in excess by 15 mol % in total.
Such ferrites largely deviating from the stoichiometric compositions are hardly controllable in electrical resistance, and tend to undergo generation of the magnetic property variation and the surface property variation among particles, and hence, even by controlling other trace components, no sufficient properties can be obtained.
According to the description on p. 177 of “Iron Oxide for Ferrite (Ferrite You Sanka Tetsu, in Japanese)” by Toshiro Sumita, pp. 175 to 177, the manganese contents of the JIS first to third types of iron oxide are 0.30% by weight or less. Thus, the content of manganese as a concomitant impurity in the generally commercially available iron oxide can be found to be 3000 ppm or less.
Recently, in two-component electrophotographic developers, acceleration of the development performance and full-color development are strongly required. Among such requirements, higher durability and higher reliability are demanded, and no generation of image defects is also demanded.
Examples of the factors for improving the durability of electrophotographic developers include the light specific gravity of the carrier. Additionally, for the purpose of attaining high reliability, small variations among carrier particles are required for various properties such as the electrical resistance properties, magnetic properties and surface property. For the purpose of preventing the generation of image defects, it is required that the carrier be high in electrical resistance and no leakage be generated even in a high electric field. None of the ferrite carriers described in the above-described patent documents meets these requirements.